I'd like to think it's something a little more twisted.
When a BA falls to the black rage they are said to re-live the last moments of sanguinius in his battle against Horus. I think that because Mephiston has fallen and managed to overcome the black rage he knows the truth of what happened.
Whatever that truth may be is now eating away at him and he knows that it will eventually destroy the Blood Angels.
When a BA falls to the black rage they are said to re-live the last moments of sanguinius in his battle against Horus. I think that because Mephiston has fallen and managed to overcome the black rage he knows the truth of what happened.
Whatever that truth may be is now eating away at him and he knows that it will eventually destroy the Blood Angels.